Articles in Solar Physics from astro-ph.SR

Day: 1 May 2018

A quiescent prominence was observed at north-west limb of the Sun using different channels of Atmospheric Imaging Assembly (AIA) onboard Solar Dynamics Observatory (SDO). We report and analyse twisting/swirling motions during and after the prominence eruption. We segregate the observed rotational motions into small and large scale. Small scale rotational motions manifest in the barbs of the prominence while the large scale rotation manifests as the roll motion during the prominence eruption. We noticed that both footpoints of the prominence rotate in the counter-clockwise direction. We propose that similar sense of rotation in both footpoints leads to prominence eruption. The prominence erupted asymmetrically near the southern footpoint which may be due to uneven mass distribution and location of the cavity near southern footpoint. Furthermore, we study the swirling motion of the plasma along different circular paths in the cavity of the prominence after the prominence eruption. The rotational velocities of the plasma moving along different circular paths are estimated to be $\sim$ 9-40 km s$^{-1}$. These swirling motions can be explained in terms of twisted magnetic field lines in the prominence cavity. Finally, we observe the twist built up in the prominence, being carried away by the coronal mass ejection (CME) as seen in the Large Angle Spectrometric Coronagraph (LASCO) onboard Solar and Heliospheric Observatory (SOHO).

The solar activity in the current, that is, the 24-th, sunspot cycle is analyzed. Cyclic variations in the sunspot number (SSN) and radiation fluxes in various spectral ranges have been estimated in comparison with the general level of the solar radiation, which is traditionally determined by the radio emission flux $F_ {10.7} $ at a wavelength of 10.7 cm (2.8 GHz). The comparative analysis of the variations in the solar constant and solar indices in the UV range, which are important for modeling the state of the Earth’s atmosphere, in the weak 24th cycle and strong 22nd and 23rd cycles showed relative differences in the amplitudes of variations from the minimum to the maximum of the cycle. The influence of the hysteresis effect between the activity indices and $F_ {10.7} $ in the 24-th cycle, which is considered here, makes it possible to refine the forecast of the UV indices and solar constant depending on the quadratic regression coefficients that associate the solar indices with $F_ {10.7} $ depending on the phase of the cycle.

The Sun is the major source of heat and light in our solar system. The solar cycle is the 11-year cycle of solar activity that can be determined by the rise and fall in the numbers and surface area of sunspots. Solar activity is associated with several factors including radio flux, solar irradiance, magnetic field, solar flares, coronal mass ejections, and solar cycles. This study attempts to determine the Sun’s activity specifically for the coronal mass ejection, its trend during solar cycle 23, and its apparent difference. A time series analysis was used to measure the CME data for larger cases and to see the apparent difference and trends of the CMEs. The result shows that a decreasing trend of coronal mass ejection from the year 1996 to 2016. It is therefore concluded that the coronal mass ejection data are normally distributed while coronal mass ejections are distributed and curved normally as fluctuation was found in the intensity of the disturbed storm time index as the number of great geomagnetic storms undeniably increased in the ascending and descending phases of the cycle. This reveals that eventhough the Sun has cycles and trends, it shows its inherent characteristics. The Sun still possess getting more dynamic through time which showcases through the limited parameters involved in this study.

Coronal mass ejections (CMEs) have become one of the key indicators of solar activity, especially in terms of the consequences of the transient events in the heliosphere. Although CMEs are closely related to the sunspot number (SSN), they are also related to other closed magnetic regions on the Sun such as quiescent filament regions. This makes CMEs a better indicator of solar activity. While sunspots mainly represent the toroidal component of solar magnetism, quiescent filaments (and hence CMEs associated with them) connect the toroidal and poloidal components via the rush-to-the-pole (RTTP) phenomenon. Taking the end of RTTP in each hemisphere as an indicator of solar polarity reversal, it is shown that the north-south reversal asymmetry has a quasi-periodicity of 3-5 solar cycles. Focusing on the geospace consequences of CMEs, it is shown that the maximum CME speeds averaged over Carrington rotation period show good correlation with geomagnetic activity indices such as Dst and aa.

The temporal recurrence of micro-flare events is studied in a time interval before and after of major solar flares. Our sample is based on the x-ray flare observations by the Geostationary Operational Environmental Satellite (GOES) and Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI). The analysed data contains 1330/301 M- and X-class GOES/RHESSI energetic solar flares and 4062/4119 GOES/RHESSI micro-flares covering the period elapsed since 2002. The temporal analysis of recurrence, by Fast Fourier Transform (FFT), of the micro-flares shows multiple significant periods. Based on the GOES and RHESSI data, the temporal analysis also demonstrates that multiple periods manifest simultaneously in both statistical samples without any significant shift over time. In the GOES sample, the detected significant periods are: $11.33$, $5.61$, $3.75$, $2.80$ and $2.24$ minutes. The RHESSI data shows similar significant periods at $8.54$, $5.28$, $3.66$, $2.88$ and $2.19$ minutes. The periods are interpreted as signatures of standing oscillations, with the longest period ($P_{1}$) being the fundamental and others as higher harmonic modes. The period ratio of the fundamental and higher harmonics ($P_{1}/P_{N}$) is also analysed. The standing modes may be signatures of global oscillations of the entire solar atmosphere encompassing magnetised plasma from photosphere to corona in active regions.

We explore a reduced Babcock-Leighton (BL) dynamo model based on delay differential equations using numerical bifurcation analysis. This model reveals hysteresis, seen in the recent mean-field dynamo model and the direct numerical simulations of turbulent dynamos. The BL model with ‘magnetic noise’ as an additional weak-source of the poloidal field recovers the solar cycle every time from grand minima, which BL source alone cannot do. The noise-incorporated model exhibits a bimodal distribution of toroidal field energy confirming two modes of solar activity. It also shows intermittency and reproduces phase space collapse, an experimental signature of the Maunder Minimum. The occurrence statistics of grand minima in our model agree reasonably well with the observed statistics in the reconstructed sunspot number. Finally, we demonstrate that the level of magnetic noise controls the duration of grand minima and even has a handle over its waiting period, suggesting a triggering effect of grand minima by the noise and thus shutting down the global dynamo. Therefore, we conclude that the ‘magnetic noise’ due to small-scale turbulent dynamo action (or other sources) plays a vital role even in Babcock-Leighton dynamo models.